The use of superconducting devices, and particularly Josephson tunnelling devices, in sampling or A/D circuits is already known in the art. Use of a Josephson device provides a very sensitive detector offering the possibility of very fast sampling speeds because such a device is capable of extremely fast switching speed between two stable states and because the device responds to extremely small magnetic fields. U.S. Pat. No. 4,401,900 shows a Josephson sampling technique with a time resolution of 5 picoseconds and a sensitivity of 10 microvolts. The time resolution of the described sampling system is extendable to the subpicosecond domain, limited ultimately by the intrinsic switching speed of the Josephson device used as the sampling gate. In principle, the switching speed can be as fast as 0.09 picoseconds. Josephson sampling techniques are not restricted to only those waveforms produced in a cryogenic environment. Rather, they can be used to measure waveforms from various sources, such as x-rays, optical photons or electrical waveforms produced by room-temperature sources, if a suitable interface is available. Examples of such interfaces are described in the co-pending patent applications Serial No. 796,841, entitled "Room Temperature to Cryogenic Electrical Interface" filed on Nov. 12, 1985, U.S. Pat. No. 4,739,633 and Ser. No. 796,842, entitled "Open Cycle Cooling of Electrical Circuits" filed on Nov. 12, 1985 now U.S. Pat. No. 4,715,189.
The Josephson sampling system described in U.S. Pat. No. 4,401,900 comprises a superconductive monitor gate, such as a single Josephson device, which has at least two states distinguishable from one another and which is sensitive to the unknown waveform or signal to be sampled. Switching means, which includes the source of the unknown signal, a source of timing pulses, and a source of a bias signal, changes the state of the monitor gate by a proper combination of the above signals. A timing means is provided to establish both a timing reference and an accurate sampling delay time. The timing means includes a pulse generator for providing very short sampling pulses, delay lines, and a source of trigger pulses. The sampling system also has noise elimination means to ensure the accuracy of the sample at any given instant of time and a display to indicate the unknown waveform.
Sampling systems, however, are inadequate to accurately measure discontinuities of network connections and to determine parameters of certain networks and devices. In such applications, time domain reflectometers, which comprise sampling circuitry with a step or pulse source, are needed. Such a device usually supplies a pulse of a very short duration or a step with a very short rise time. The shorter the rise time, the higher the time accuracy and the finer the details which can be measured by the sampling circuitry. The only time domain reflectometer system (TDR system) that is known to the applicant as being available commercially is manufactured by Tektronix, Inc. of Beaverton, Oregon as a plug-in module to its 7000 series oscilloscope. The TDR system consists of the sampling system plus a separate pulse generator and has a system rise time of more than 40 picoseconds.
One problem of existing TDR systems, such as the one described above, is the relatively long system rise time which is inadequate for displaying rapidly changing waveforms. A second problem is that existing TDR systems have the sampling circuitry separate from the pulse generator. Thus, if an existing superconducting sampling system is utilized to overcome the rise time problem, the pulse generator of the TDR system would be separate from, and only bonded to, the integrated circuit chip upon which the sampling circuitry is formed. Such a bond, however, has been shown to have a reliability risk, as well as performance limitations. Further, it has been shown by the aforementioned copending applications that the thermal, mechanical and electrical constraints that must be satisfied in order to perform superconducting sampling of room-temperature devices can be obviated by a monolithic chip having all the particular circuitry and high performance transmission lines formed thereon.